1. Field of the Invention
The present invention generally relates to lubrication systems for internal combustion engines and, more particularly, is concerned with lubrication systems providing pressurized oil to the engine when a mechanical oil pump is not fully operational.
2. Description of the Prior Art
Internal combustion engines, principally of the gasoline fueled varieties, have been the primary motive devices behind automobiles for over eighty years. During this time, the automobile engine has benefited from improvements too numerous to list. However, although at first glance a modern reciprocating engine would appear radically different from an early engine, such as the engine installed in a Ford Model T, once stripped down to their cores the two engines would have nearly identical components. In all internal combustion engines, fundamental moving components such as pistons, connecting rods, camshafts, crankshafts, valves and so on, must contend with frictional forces.
Engine friction has historically been mitigated by a lubrication system which includes a mechanical oil pump to force-feed oil throughout the engine. Nevertheless, engine wear, represented by such things as worn piston rings and leaky valves, will generally limit the life span of an engine. At some point, if the wear on an engine is left unchecked, the engine will cease functioning completely. The life of an engine can be prolonged, however, if certain extraordinary periods of wear are alleviated by the lubrication system. These periods of wear are not normally serviced by the mechanical oil pump.
The most critical time for engine wear occurs between the initiation of starter motor cranking and the pressurization of the engine oil circuit by the mechanical oil pump. In summary, engine wear is most extensive during periods when frictional components are not being adequately lubricated, i.e., when the oil pressure induced by the mechanical oil pump is beneath some nominal level.
Frictional damage also arises inside turbochargers. An exhaust driven turbocharger contains a rotor, driven by exhaust gas, which spins at speeds exceeding 30,000 r.p.m. This figure translates into the equivalent of 500 revolutions per second by the turbocharger rotor. The rotor spins on a shaft, which is indirectly connected to the rotor by a center bearing. The center bearing serves to absorb the severe frictional forces caused by the tremendous angular velocities of the spinning rotor.
After the engine ignition is turned off, the rotor continues to spin at a high speed without the benefit of engine oil pressure. This period of time is appropriately referred to as "spin-down". Besides the loss of pressure at the center bearing during spin-down, the bearing also loses a medium of heat exchange. The oil on the center bearing will normally transfer the heat which has been absorbed by the bearing from the exhaust gases carried by the turbocharger rotor. However, during spin-down the oil remaining around the bearing surface will burn, depositing an abrasive coke layer around the surface and thereby causing premature wear. Since turbocharger life is primarily measured by the condition of the center bearing, the life of the turbocharger can be extended if the center bearing is provided adequate oil pressure during spin-down.
Clearly, because there is a significant payback in engine life, many people familiar with lubrication system technology have been actively working to prolong engine and turbocharger life by minimizing the wear on frictional components during the periods discussed above. The typical approach to pressurizing the lubrication system during these periods is to add an external lubrication circuit to the engine. The external circuit includes an electrically operated oil pump, which operates during specific periods when the mechanical oil pump is not fully operational. The patents issued to Sundles, et al. (U.S. Pat. No. 4,628,877) and Murther (U.S. Pat. No. 4,531,485) are two representative examples of such lubrication systems incorporating electrically operated oil pumps.
Sundles, et al., discloses an electrically operated oil pump, external to the engine, having an inlet connected by a suction hose to the oil sump of the engine. At the outlet of the electric pump, a one-way check valve prevents pressure leakage between the internal lubrication circuit and the external lubrication circuit. A bypass valve connects the electric pump outlet to the pump inlet to prevent pump pressure overload when the pump is running and the one-way check valve is closed. Two conduits connect the outlet of the one-way check valve to the engine and the turbocharger. A first time-delay relay connected to the ignition system energizes the electric pump after the ignition is turned on, thus lubricating the engine during cranking. A second time-delay relay energizes the electric pump after the ignition is turned off, thus lubricating the turbocharger during spin-down.
Sundles, et al. exemplifies one of a number of related lubrication systems which perform satisfactorily, but also for which several areas of improvement have been identified. In such prior technology lubrication systems, oil from the electrically operated pump enters the engine by way of a T-fitting placed between the engine and a conventional oil pressure sender. Typically, sender units are not readily accessible, and even where a work area for a unit is convenient, there are other considerations in choosing not to use the sender unit location as an oil inlet.
For instance, since there is a large variety of sender unit threadings, threadings between the sender unit and the T-fitting may not match. In addition, near the sender unit location on the engine, the space for attaching the oil conduit and the fittings, which form a part of the external lubrication circuit, is usually limited. More importantly, because oil sender units are usually located at the midpoint of engine oil galleries, oil disbursement from such a location to the larger, lower oil galleries is not as thorough as the oil distribution made by the mechanical oil pump located near the oil filter at the bottom of the engine.
In addition, the T-fitting causes oil to flow in two directions, often forcing air down into crankshaft main and connecting rod bearings. The resulting oil starvation at these critical components can produce complete engine breakdown. Further, when oil is directed into the engine at the oil sender location, oil for the external lubrication circuit is pumped out of the oil sump and back into the engine without filtration, thereby depositing unwanted grit into the upper engine. As a final shortcoming to be noted, if the oil conduit between the electric pump outlet and the engine is disconnected or broken while the engine is running, engine oil is immediately evacuated from the engine through the T-fitting thereby causing the engine to seize.
As another example of a lubrication system having an external lubrication circuit, Murther shows an electrical oil pump running in parallel with the mechanical oil pump. Oil enters the mechanical and electrical oil pumps through an oil outlet pipe connected to the oil sump. A one-way check valve at the outlet of each pump prevents oil from back-flowing between the pumps. The check valve outlets are joined and enter the oil filter through a single conduit. Oil from the oil filter returns to the engine though an oil inlet pipe.
The Murther lubrication system has at least three serious disadvantages. First, the Murther electrical oil pump is timed to pump oil only after the starter motor is activated, and thus, the lubrication system is not fully pressurized at the beginning of the critical cranking period. Second, the oil filter used in Murther has a single inlet into which oil is pumped from the mechanical and electrical oil pumps. Such a single inlet is an awkward means of connecting the oil filter to the two oil pumps, since the configuration shown in Murther either requires a special type of oil filter distinct from the standard "spin-on" oil filter, or it requires both the mechanical and the electrical pumps to be located inside the engine. Third, the Murther invention does not provide for oiling a turbocharger center bearing during spin-down.
Consequently, a need exists for still further improvement in engine and turbocharger lubrication systems, particularly in routing and filtering oil pumped from the oil sump into the engine block by an electrically operated oil pump.